Run pyupgrade on core/groupby (#6351)
Run pyupgrade on core/groupby (#6351)

Co-authored-by: pre-commit-ci[bot] <3ea08453efb076ca910552532a53973a6527df40@users.noreply.github.com>
markelg/xray,jhamman/xarray,pydata/xarray,markelg/xray,pydata/xarray,jhamman/xarray,markelg/xray,pydata/xarray,jhamman/xarray